Electroscope

The electroscope is an early scientific tool used to detect electric charge on an object.  It detects charge by the movement of a test object due to the Coulomb electrostatic force (the force of electricity pulling or pushing it).  The amount of charge on an object depends on its size and shape, and on the voltage (electrical "pressure") between it and its surroundings. Electroscopes need a lot of voltage to work, usually between a few hundred to a few thousand volts, so they are used with strong sources of static electricity, such as electrostatic machines. An electroscope can only give a general idea of the amount of charge an object has; a tool that measures the exact electric charge is called an electrometer.

The electroscope was the first tool for measuring electricity to be invented. William Gilbert, a British doctor, invented the first electroscope (called the versorium) around the year 1600, using a metal needle that could turn. a translation by  P. Fleury Mottelay of William Gilbert (1600) Die Magnete, London Two other classic types of electroscopes are still used today to teach about static electricity: the pith-ball electroscope and the gold-leaf electroscope.  Another type of electroscope is used in a radiation dosimeter, a special tool for measuring radiation. Electroscopes also helped Austrian scientist named Victor Hess discover cosmic rays.

 Pith-ball electroscope 

In 1731, Stephen Gray created a simple electroscope using a hanging thread. The thread would move towards any charged object nearby. This was the first improvement on the original design by William Gilbert.

In 1754, British teacher John Canton invented the pith-ball electroscope. It has one or two lightweight balls, originally made of a plant material called pith, p. 609 hanging from a hook by a silk or linen string. In 1770, Tiberius Cavallo made an electroscope like Canton's but with silver wires instead of string. Modern versions use plastic balls. To test if an object has a charge, you bring it near the ball. If the object is charged, the ball will be attracted to it and move.

The pith ball is attracted because the charged object polarizes the ball's atoms, meaning it pulls their electrons to one side, creating a charge. Paul E. Tippens,  Electric Charge and Electric Force, Powerpoint presentation, pp. 27–28, 2009, S. Polytechnic State Univ.  on DocStoc.com website  Atoms, the building blocks of all matter, are made of electrically charged particles called protons, neutrons, and electrons. Each atom has a positively charged nucleus made of protons and neutrons, with a cloud of negatively charged electrons surrounding it. The pith is a nonconductor, which means the electrons in the ball can't move around and make a current, but they can move a little within the atoms, making a charge. As shown in the diagram, a positively charged object (B) is brought near the pith ball (A), the negative electrons (blue minus signs) in each atom  will get pulled to the side of the atom closer to the object. The positively charged nuclei (red plus signs) will be pushed to the side farther away.  Since the electrons in the pith ball are closer to the object (C), they are pulled more than the nuclei are pushed, meaning the pulling force wins and the atom moves towards the object.  On their own, the atoms move so little that you can't see it without a microscope, but the pith ball has millions of atoms, so they all pull a little bit and it adds up to enough pulling that you can see it.

Because the opposite charged particles within both objects are attracted to each other, some of the charges jump from the object to the pith ball, and from the pith ball to the object. This leads to the object losing some positive charge and transferring it to the pith ball, which becomes positively charged itself. Now, instead of getting pulled towards any charged object, it gets pulled towards negative charges and gets pushed away from positive charges, meaning you can use the pith ball to figure out not only if something has a charge, but also if it's positive or negative. 

Electroscopes with two balls, as opposed to one, have an advantage in that you can tell if they are charged. If you use the same process as mentioned previously, both balls will have the same charge, and therefore will push each other away and hang in an upside down 'V' shape. With the one ball electroscope, the system acts the same regardless of whether it is charged or not, until there is a different charged object that comes near it, whereas with the two ball electroscope, there is a visible difference. Another advantage is that you can tell the magnitude of the charge because the farther they push each other away, the more they are charged.

 Gold-leaf electroscope 
Abraham Bennet, a British priest and physicist, invented the gold leaf electroscope in 1787.  It was more sensitive and precise than earlier types like the pith-ball electroscope.*[Anon.] (2001) "Electroscope", Encyclopaedia Britannica It has a straight metal rod, usually made of brass, with two thin, flexible strips of gold leaf hanging from the bottom.  A disk- or ball-shaped metal piece, where you put the charged object, sits at the top of the rod. The gold strips are stuck inside a glass jar, usually with a metal bottom that conducts electricity, to protect them from getting pushed by moving air. Often there are grounded metal plates in the jar next to the leaves.  These plates protect the delicate gold strips from too much charge; if they touch the plates, the charge goes away before the leaves can tear.  They also help the instrument work better by capturing any electricity that might escape. In very precise tools, the air was sometimes removed from the jar completely to stop the charge from leaking away.

When a charged object touches the metal top piece, the gold leaves push away from each other in an upside-down 'V'. This is because the charge from the object goes through the top piece and the metal rod to the gold strips, charging them. Since the strips both get the same kind of charge, they push each other away. If you touch the top piece with your finger, the charge leaves through your body into the ground and the gold strips stop pushing each other away.

You can also charge an electroscope without touching it with a charged object. This is called charging by electrostatic induction. When a charged object is held near the top of the electroscope, the strips spread apart, even though the charged object isn't touching the top piece. The electric field from the object forces a charge of the same kind the object has into the rod and strips. The strips end up with the same type of charge, so they push each other away.  The opposite charge collects in the top piece, near the charged object.

If you touch the electroscope with your finger while the charged object is nearby, the same-kind charge in the strips moves through your body into the ground, but the opposite-kind charge in the top piece stays where it is. When you take your finger away, the electroscope is left with an opposite overall charge. The strips hang down at first because the charge is stuck at the top, near the charged object. When you move the object away, the charge spreads out to the strips, causing them to push each other apart again. 

 Related pages 

 Electrical measurements
 Electrostatic fieldmeter
 Faraday cup electrometer
 Radiation

Footnotes

 Other websites 




Category:Electrostatics
Category:Electrical instruments
Category:Measuring instruments
Category:Historical scientific instruments